Project Summary Non-alcoholic fatty liver disease represents a group of conditions associated with excessive lipid accumulation in hepatocytes. Hepatic steatosis affects almost 30% of adults and imposes a major health burden on American society. Non-alcoholic fatty liver disease increases morbidity and mortality associated with injury or physiological stress, and thus is a concern for all medical disciplines. Our preliminary studies have identified a novel molecular pathway and novel cellular mechanisms that couple long-term energy supply to steatosis and injury. Our studies demonstrate for the first time that a member of the leucine-rich repeat-containing G protein- coupled receptors, LGR4, is expressed in hepatocytes. R-spondin family proteins were recently identified as ligands for LGR4. Our preliminary data also indicate that R-spondin1 is produced by hepatocytes and that R- spondin1-LGR4 signaling is present in the liver. Distinct from classical G protein-coupled receptors which act via G proteins, hepatic LGR4 functions mainly through Wnt/?-catenin signaling. Further, preliminary studies indicate that the mechanistic target of rapamycin complex 1 (mTORC1) in liver couples long-term nutrient status to R-spondin1-LGR4 expression. Steatosis down regulates hepatic R- spondin1/LGR4 and renders hepatic tissue more vulnerable to injury. We propose to investigate the function of the hepatic R-spondin1-LGR4 system using cell biological and transgenic techniques. Completion of this proposal will advance a completely new area of hepatic physiology and will provide novel insights into to liver injury.